package day13;

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

//跳石板
public class oj2 {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        int n = in.nextInt();
        int m = in.nextInt();
        int[] step = new int[m + 1];
        for(int i = 0; i < m+1;i++) {
            step[i] = Integer.MAX_VALUE;
        }
        step[n] = 0;
        for(int i = n; i < m;i++) {
            if(step[i]==Integer.MAX_VALUE) {
                continue;
            }
            List<Integer> list = div(i);
            for(int j:list) {
                if(i + j <= m && step[i+j] != Integer.MAX_VALUE) {
                    step[i+j] = Math.min(step[i+j],step[i]+1);
                } else if(i+j <= m) {
                    step[i+j] = step[i]+1;
                }
            }

        }
        if(step[m] == Integer.MAX_VALUE) {
            System.out.println(-1);
        } else {
            System.out.println(step[m]);
        }
    }

    //判断约数
    public static List<Integer> div(int num) {
        List<Integer> list = new ArrayList<Integer>();
        for(int i = 2;i*i <= num;i++) {
            if(num%i == 0){
                list.add(i);
                if(num/i != i) {
                    list.add(num/i);
                }
            }
        }
        return list;
    }
}
